High-strengh steel sheet and method sheet for manufacturing the same

ABSTRACT

A high-strength steel sheet having good formability and methods for manufacturing. The high-strength steel sheet has a chemical composition including, in mass %, C: 0.010% to 0.080%, Si: 0.05% or less, Mn: 0.10% to 0.70%, P: 0.03% or less, S: 0.020% or less, Al: 0.005% to 0.070% and N: 0.0120% to 0.0180%, the balance being Fe and inevitable impurities. A content of nitrogen in the form of solute nitrogen is 0.0100% or more, average ferrite grain size of the steel sheet is 7.0 μm or less, the density of dislocations at a depth of  1/4  sheet thickness from the surface of the steel sheet is 4.0×10 14  m −2  to 2.0×10 15  m −2 , and the tensile strength and the elongation in the transverse direction, perpendicular to the rolling direction, after aging treatment is 530 MPa or more and 7% or more respectively.

TECHNICAL FIELD

The present disclosure relates to high-strength steel sheets which are suited as can-making materials used in the production of food cans and beverage cans, and to methods for manufacturing such steel sheets. The high-strength steel sheets of the present disclosure exhibit highly excellent formability and can be suitably used for the manufacturing of easy-open-ends (EOEs) and welded can bodies.

BACKGROUND ART

Steel sheets called DR (double reduced) steels are sometimes used as can-making steel sheets for the production of beverage cans and food cans and are formed into parts such as lids, bottoms and three-piece can bodies. In the manufacturing of DR steels, the steel sheets are cold rolled again after annealing. The DR steels can be easily reduced in thickness while increasing hardness as compared to SR (single reduced) steels whose production involves only temper rolling with a small rolling reduction.

In recent years, considerations for the reduction of environmental load and the saving of cost require that the amount of steel sheets used in beverage cans and food cans be reduced. This trend has led to a greater demand for the use of DR steels as can-making steel sheets in order to facilitate the thinning of steel sheets.

With the hardness being increased as a result of work hardening, the DR steels generally have low formability. It is therefore necessary that the formability of the DR steels be improved in order for the DR steels to be suitably used as can-making steel sheets. For example, Patent Literatures 1 and 2 propose DR steels having improved formability.

Patent Literature 1 proposes DR steels characterized in that the steel contains, in mass %, C: 0.02% to 0.06%, Si: 0.03% or less, Mn: 0.05% to 0.5%, P: 0.02% or less, S: 0.02% or less, Al: 0.02% to 0.10% and N: 0.008% to 0.015%, the balance being Fe and inevitable impurities, the amount of solute N (Ntotal—NasAlN) in the steel sheet is 0.006% or more, the total elongation in the rolling direction after aging treatment is 10% or more, the total elongation in the sheet width direction after aging treatment is 5% or more, and the average Lankford value after aging treatment is 1.0 or less.

Patent Literature 2 proposes high-strength thin steel sheets with excellent flangeability for welded cans characterized in that the steel contains, in mass %, C: more than 0.04% and 0.08% or more, Si: 0.02% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.05% or less, Al: 0.1% or less and N: 0.005 to 0.02%, the total of solute C and solute N dissolved in the steel sheet satisfies 50 ppm≦solute C+solute N≦200 ppm, the amount of solute C in the steel sheet is 50 ppm or more and the amount of solute N in the steel sheet is 50 ppm or more, the balance of the composition being Fe and inevitable impurities.

CITATION LIST Patent Literature

PTL 1: WO 2008/018531

PTL 2: Japanese Unexamined Patent Application Publication No. 2002-294399

SUMMARY Technical Problem

However, the above techniques in the art have the following problems.

The technique described in Patent Literature 1 does not necessarily realize good formability depending on conditions such as the number of steps in the formation of rivets in EOE cans. Further, the technique described in Patent Literature 1 does not attain sufficient workability such as flangeability for three-piece cans.

In the technique described in Patent Literature 2, rivet formability required for the production of EOE cans is insufficient. Further, the technique entails prolonged over-aging treatment in order to decrease the amount of solute C, causing a decrease in production efficiency.

The present disclosure has been made in light of the circumstances discussed above. To solve the problems in the art described hereinabove, an object of the disclosure is to provide high-strength steel sheets having good formability (workability) and strength and methods for manufacturing such steel sheets.

Solution to Problem

To achieve the above object, the present inventors carried out extensive studies. As a result, the present inventors have found that the optimization of chemical composition of steel, hot rolling conditions, cold rolling conditions, annealing conditions and secondary cold rolling conditions (DR conditions) attains a tensile strength of 530 MPa or more and an elongation of 7% or more in the transverse direction after aging treatment. Further, the present inventors have found that the average ferrite grain size and the density of dislocations at ¼ sheet thickness contribute to the satisfaction of the above tensile strength and elongation. The present disclosure has been completed based on the findings. Exemplary embodiments include the following aspects:

-   -   (1) A high-strength steel sheet having a chemical composition         including, in mass %, C: 0.010% to 0.080%, Si: 0.05% or less,         Mn: 0.10% to 0.70%, P: 0.03% or less, S: 0.020% or less, Al:         0.005% to 0.070% and N: 0.0120% to 0.0180%, the balance being Fe         and inevitable impurities, nitrogen present as solute nitrogen         having a content of 0.0100% or more in the N content, an average         ferrite grain size being 7.0 μm or less, a density of         dislocations at a depth of ¼ sheet thickness from the surface         being 4.0×10¹⁴ m⁻² to 2.0×10¹⁵ m⁻², a tensile strength and an         elongation in the transverse direction perpendicular to the         rolling direction after aging treatment being 530 MPa or more         and 7% or more.

(2) A method for manufacturing the high-strength steel sheet described in (1) including a hot rolling step of heating a slab at a heating temperature of 1180° C. or more, rolling the slab with a hot rolling finish temperature of 820 to 900° C. and coiling the sheet at a coiling temperature of 640° C. or less, a primary cold rolling step of pickling the hot-rolled steel sheet and cold rolling the sheet with a rolling reduction of 85% or more, an annealing step of annealing the primarily cold-rolled steel sheet at 620° C. to 690° C., and a secondary cold rolling step of secondarily cold rolling the annealed steel sheet with a rolling reduction of 8 to 20%.

Advantageous Effects

The high-strength steel sheets of the disclosed embodiments have a specific chemical composition, an average ferrite grain size of 7.0 μm or less, and a density of dislocations at a depth of ¼ sheet thickness from the surface of 4.0×10¹⁴ m⁻² to 2.0×10¹⁵ m⁻². With this configuration, the steel sheets attain a tensile strength of 530 MPa or more and an elongation of 7% or more in the transverse direction after aging treatment.

The high-strength steel sheets of the present disclosure have high formability as described above, and may be suitably used in applications in which the steel sheets are formed into rivets or are flanged. In particular, the inventive high-strength steel sheets have a tensile strength of 530 MPa or more. This sufficient strength allows the sheets to form quality can bodies or lids even when the sheet thickness is reduced compared to the conventional materials. The reduction of sheet thickness saves resources and costs.

The high-strength steel sheets of the present disclosure, which are excellent in formability and strength, are not only used in various types of metal cans but are also expected to find use in a wide range of applications such as battery interior cases, various home appliance and electrical parts, and automobile parts.

DESCRIPTION OF EMBODIMENTS

Hereinbelow, some exemplary embodiments of the present disclosure will be described. However, the scope of the present disclosure is not limited to such embodiments.

The high-strength steel sheets of the present disclosure have a specific chemical composition, and the average ferrite grain size and the density of dislocations at ¼ sheet thickness are controlled to fall in the specific ranges. By virtue of this configuration, the inventive high-strength steel sheets attain excellent formability while exhibiting high strength. In the following, the chemical composition, the average ferrite grain size, the density of dislocations at ¼ sheet thickness, the quality (high strength, high formability) of the high-strength steel sheets, and the methods for manufacturing the high-strength steel sheets will be sequentially described.

<Chemical Composition>

The high-strength steel sheet of the disclosed embodiments has a chemical composition including, in mass %, C: 0.010% to 0.080%, Si: 0.05% or less, Mn: 0.10% to 0.70%, P: 0.03% or less, S: 0.020% or less, Al: 0.005% to 0.070% and N: 0.0120% to 0.0180%, the balance being Fe and inevitable impurities. Of the N content in the steel, 0.0100% or more is the content of solute nitrogen. These components will be described below. In the following description, “%” indicates “mass %”.

C: 0.010% to 0.080%

Carbon is an element that contributes to increasing the strength of the steel sheets. By limiting the C content to 0.010% or more, the steel can attain a tensile strength of 530 MPa or more in the transverse direction after aging treatment. If the C content exceeds 0.080%, the elongation in the transverse direction after aging treatment falls to below 7% and the steel sheets exhibit poor flangeability or rivet formability. Thus, the C content needs to be limited to 0.080% or less. To ensure good flangeability and rivet formability, it is preferable that the C content be less than 0.040%. Because the average ferrite grain size is reduced with increasing C content, it is preferable that the C content be 0.020% or more in order to ensure that the steel sheets will exhibit high strength.

Si: 0.05% or less

If the steel sheets contain an excessively large amount of silicon, the element is enriched at the surface to cause a decrease in the surface treatment properties of the steel sheets. Consequently, the corrosion resistance of the steel sheets is reduced. Thus, the Si content needs to be limited to 0.05% or less. The Si content is preferably 0.03% or less.

Mn: 0.10% to 0.70%

Manganese has an effect of enhancing the hardness of the steel sheets by solution strengthening. Further, manganese forms MnS and thereby effectively prevents the decrease in hot ductility (casting properties) ascribed to sulfur present in the steel. To obtain these effects, the Mn content needs to be limited to 0.10% or more. Because manganese has an effect of reducing the grain size, it is preferable that the Mn content be 0.20% or more. Further, manganese decreases the rate of the diffusion of nitrogen and thereby inhibits the formation of AlN to ensure the presence of nitrogen as solute. Thus, the addition of manganese is effective particularly when the tensile strength is to be increased to 590 MPa or more. In view of these facts, it is more preferable that the Mn content be more than 0.50%. The Mn content is limited to 0.70% or less because any excessive addition of manganese not only results in the saturation of the above effects but also causes a marked decrease in elongation.

P: 0.03% or less

Abundant phosphorus causes a decrease in formability by excessive hardening or central segregation. Further, the presence of a large amount of phosphorus causes a decrease in corrosion resistance. Thus, the P content is limited to 0.03% or less. The P content is preferably 0.02% or less.

S: 0.020% or less

Sulfur forms sulfides in the steel to cause a decrease in the hot ductility of the steel sheets. Thus, the S content is limited to 0.020% or less. The S content is preferably 0.015% or less.

Al: 0.005% to 0.070%

Aluminum is an element added as a deoxidizer. To obtain this effect, the Al content needs to be limited to not less than 0.005%. Aluminum decreases the amount of solute nitrogen in the steel by forming AlN with nitrogen. The decrease in the amount of solute nitrogen results in a decrease in the strength of the steel sheets. Thus, the Al content is limited to 0.070% or less. To ensure that the amount of solute nitrogen will be stably 0.0100% or more, it is preferable that the Al content be 0.020% or less, and more preferably 0.018% or less.

N: 0.0120% to 0.0180%, Solute N: 0.0100% or more

Nitrogen present in the form of solute nitrogen contributes to increasing the strength of the steel sheets. When solute nitrogen is present in 0.010% or more, the introduction of dislocations during secondary cold rolling is facilitated and consequently the balance between high strength and formability is enhanced. To obtain these effects, the content of nitrogen in the form of solute nitrogen needs to be limited to 0.0100% or more. The solute N content is more preferably 0,0120% or more. To ensure that the solute N content will be 0.0100% or more, the N content needs to be limited to 0.0120% or more. The N content is preferably more than 0.0130%. To ensure that the solute N content will be stably 0.0120% or more, it is preferable that the formation of AlN during the manufacturing steps be suppressed by one or a combination of any of (1) controlling the Mn content to more than 0.50%, (2) controlling the coiling temperature during hot rolling to 640° C. or less, preferably 600° C. or less, and more preferably 580° C. or less, and (3) controlling the annealing temperature to 690° C. or less, and more preferably less than 680° C. When the tensile strength is increased to 600 MPa or more in order to further increase the strength or to further reduce the thickness of cans, it is preferable that all the above three conditions be combined so that the steel will exhibit high formability with the elongation being 10% or more. On the other hand, adding a large amount of nitrogen causes a decrease in elongation and consequently results in decreases in rivet formability and flangeability. Thus, the N content is limited to 0.0180% or less. The N content is preferably 0.0170% or less. With the N content being in the above range, the content of nitrogen in the form of solute nitrogen is 0.0180% or less.

The balance after the deduction of the above essential components is iron and inevitable impurities.

<Average Ferrite Grain Size: 7.0 μM or Less>

In the steel sheets which satisfy the above chemical composition and also have a specific density of dislocations at a depth of ¼ sheet thickness, the balance between high strength and formability is enhanced by reducing the size of ferrite grains so that the average ferrite grain size will be 7.0 μm or less. Further, the reduction in average ferrite grain size provides another advantage that the roughening of skin after working is prevented. In view of this, the average ferrite grain size is preferably 6.5 μm or less. The average ferrite grain size is a value measured by the method described in EXAMPLES. With the size of ferrite grains after annealing being finer, the introduction of dislocations during secondary cold rolling is facilitated more efficiently and consequently high strength is obtained even with a smaller rolling reduction. Consequently, the balance between high strength and formability is further enhanced. The average ferrite grain size after secondary cold rolling is reduced compared to that after annealing (before the secondary cold rolling). In view of this fact, it is more preferable that the average ferrite grain size after the secondary cold rolling be 6.0 μm or less in order to obtain the above effects. The lower limit of the average ferrite grain size is not particularly limited. If, however, the grains are excessively fine, the balance between high strength and formability is decreased. For this reason, the average grain size is preferably 1.0 μm or more. The microstructure of the inventive steel is based on ferrite and the ferrite phase represents 98 vol % or more.

<Density of dislocations at ¼ sheet thickness: 4.0×10¹⁴ m⁻² to 2.0×10¹⁵ m⁻²>

In the present disclosure, the control of the density of dislocations in the steel sheets is important in order to satisfy the strength and formability of the steel sheets at the same time. In the present disclosure, the density of dislocations at a depth of ¼ sheet thickness needs to be controlled to 4.0×10¹⁴ m⁻² or more in order to attain an increase in strength. Dislocations present in an excessively high density induce the occurrence of voids during forming and thus cause a decrease in the formability of the steel sheets. Thus, the dislocation density needs to be controlled to 2.0×10¹⁵ m⁻² or less. To control the dislocation density in the above range, in particular, it is important that the solute N content be controlled to 0.0100% or more or preferably 0.0120% or more, and the average ferrite grain size be controlled to 7.0 μm or less, preferably 6.5 μm or less or more preferably 6.0 μm or less. The density of dislocations at ¼ sheet thickness is a value measured by the method described in EXAMPLES.

<Quality>

The high-strength steel sheets of the present disclosure achieve high formability while having high strength by virtue of having the chemical composition described hereinabove and also because of the average ferrite grain size and the density of dislocations at ¼ sheet thickness being controlled to 7.0 μm or less and from 4.0×10¹⁴ m⁻² to 2.0×10¹⁵ m².

In general, it is very difficult for thin steel sheets to satisfy both high strength and high formability. The term “thin” means that the thickness is not more than 0.26 mm. According to the present disclosure, the sheet thickness can be reduced to 0.12 mm while still ensuring that high strength and high formability can be satisfied at the same time.

The term “high strength” means that the tensile strength in the transverse direction perpendicular to the rolling direction after aging treatment is not less than 530 MPa. With the tensile strength being not less than 530 MPa, sufficient strength of cans may be ensured when the steel sheets are formed into can lids or can bodies. The tensile strength is preferably 550 MPa or more, and more preferably 590 MPa or more. With the tensile strength being 550 MPa or more, high strength and high formability can be satisfied simultaneously even when the sheets are extremely thin. The phrase “extremely thin” means that the thickness is 0.18 mm or less.

By the term “high formability”, it is meant that the elongation in the transverse direction perpendicular to the rolling direction after aging treatment is 7% or more. With the elongation being 7% or more, the high-strength steel sheets of the disclosure applied to can bodies or EOE cans exhibit sufficient flangeability required for the production of can bodies or rivet formability demanded in the manufacturing of EOE cans. Higher formability is necessary when the tensile strength is as high as 550 MPa or more, and in this case it is preferable that the elongation in the transverse direction after aging treatment be 10% or more.

In the production of cans, steel sheets are frequently formed after coatings are baked on the steel sheets. In view of this, the aging treatment prior to the quality evaluation should be equivalent to such baking.

<Methods for Manufacturing High-Strength Steel Sheets>

Hereinbelow, an example of the methods for manufacturing the high-strength steel sheets of the present disclosure will be described.

The high-strength steel sheets of the present disclosure may be produced by a method including a hot rolling step, a primary cold rolling step, an annealing step and a secondary cold rolling step. These steps will be described below.

Hot Rolling Step

In the hot rolling step, a slab having the aforementioned chemical composition except the solute N content (the solute N content may be satisfied or not satisfied) is heated at a heating temperature of 1180° C. or more, rolled with the hot rolling finish temperature being 820 to 900° C., and coiled at a coiling temperature of 640° C. or less.

If the slab heating temperature is excessively low, part of AlN is not dissolved and consequently the solute N content is reduced. Thus, the heating temperature is limited to 1180° C. or more. The heating temperature is preferably 1200° C. or more. While the upper limit of the heating temperature is not particularly limited, an excessively high heating temperature may give rise to the occurrence of excessive scales, resulting in defects on the product surface. Thus, the heating temperature is preferably 1300° C. or less.

If the hot rolling finish temperature is more than 900° C., the grains in the hot-rolled sheet are coarsened and consequently the grain size in the annealed sheet is increased and the hardness of the steel sheet is decreased. Thus, the hot rolling finish temperature is limited to 900° C. or less. If the hot rolling finish temperature is less than 820° C., the rolling takes place at or below the Ar3 transformation point, and consequently the formability is decreased due to the formation of coarse grains and the remaining of deformation microstructure. Thus, the hot rolling finish temperature is limited to 820° C. or more. The hot rolling finish temperature is preferably 840° C. or more.

If the coiling temperature is more than 640° C., a large amount of AlN is formed during the coiling and consequently the amount of solute nitrogen is reduced. Further, coiling at more than 640° C. results in the coarsening of the grains in the hot-rolled sheet and thus causes the grain size after annealing to be increased. For these reasons, the coiling temperature is limited to 640° C. or less. The coiling temperature is preferably 600° C. or less, and more preferably 580° C. or less. The lower limit of the coiling temperature is not particularly limited. If, however, the coiling temperature is excessively low, a great variation in temperature is caused during cooling possibly to give rise to wide variations in tensile strength and elongation. In view of this, the coiling temperature is preferably 500° C. or more.

Primary Cold Rolling Step

In the primary cold rolling step, the hot-rolled steel sheet is pickled and primarily cold rolled with a rolling reduction of 85% or more.

The pickling conditions are not particularly limited as long as skin scales can be removed. Usual pickling methods may be used.

The grain size after annealing may be reduced and the balance between tensile strength and elongation may be enhanced by appropriately controlling the rolling reduction during the primary cold rolling. To obtain these effects, the rolling reduction is limited to 85% or more. Rolling with an excessively large reduction causes tensile strength and elongation to be widely anisotropic in plane, resulting in a decrease in formability. Thus, the rolling reduction in this step is preferably less than 91.5%.

Annealing Step

In the annealing step, the cold-rolled sheet is annealed at an annealing temperature of 620° C. or more and 690° C. or less.

To ensure formability, the microstructure should be sufficiently recrystallized during annealing. For this purpose, the annealing temperature needs to be limited to 620° C. or more. If the annealing temperature is excessively high, the average ferrite grain size is increased and the balance between tensile strength and elongation is lowered. In view of this, the annealing temperature is limited to 690° C. or less. At high annealing temperatures, AlN tends to be formed to cause a decrease in the amount of solute nitrogen. Thus, it is preferable that the annealing temperature be 680° C. or less. The annealing method is not particularly limited. From the point of view of quality uniformity, a continuous annealing method is preferable. The holding time in the annealing step is not particularly limited but is preferably not less than 5 seconds from the point of view of the uniformity in steel sheet temperature, and is preferably 90 seconds or less in order to prevent the increase in average ferrite grain size.

Secondary Cold Rolling (DR Rolling) Step

In the secondary cold rolling step, the annealed sheet is secondarily cold rolled with a rolling reduction of 8 to 20%.

The annealed steel sheet is strengthened by being subjected to the secondary rolling. Further, the thickness of the steel sheet is reduced by the secondary rolling. To increase the density of dislocations at a depth of ¼ sheet thickness from the surface and thereby to increase the strength of the steel sheet, the rolling reduction (the DR ratio) in the secondary cold rolling is limited to 8% or more. If the DR ratio is too high, the dislocation density is excessively increased and the formability is decreased. In view of this, the DR ratio is limited to 20% or less. When formability is particularly required, the DR ratio is preferably controlled to 15% or less.

The high-strength steel sheets of the present disclosure are obtained in the manner described hereinabove. The advantageous effects of the present disclosure may be still attained even when the steel sheets obtained are subjected to surface treatments such as plating and chemical conversion.

EXAMPLES

Steels A to N having the chemical compositions described in Table 1, the balance being iron and inevitable impurities, were smelted and cast into steel slabs. Under the conditions described in Table 2, the steel slabs were heated, hot rolled and pickled to remove scales. Thereafter, the steel sheets were primarily cold rolled with the primary cold rolling reductions described in Table 2, annealed in a continuous annealing furnace at the respective annealing temperatures, and subjected to secondary cold rolling (DR rolling) with the respective secondary cold rolling reductions. In this manner, steel sheets (steel sheets Nos. 1 to 22) having a sheet thickness of 0.15 to 0.26 mm were obtained. Both sides of each steel sheet obtained were plated with tin in a coating mass of 2.8 g/m² per side. The tin-plated steel sheets were subjected to evaluations of characteristics by the following methods.

Amount of Solute Nitrogen

The amount of solute nitrogen was determined by subtracting the amount of nitrogen as AlN measured by extraction analysis with 10% Br methanol, from the total amount of nitrogen.

Tensile Strength and Elongation in Transverse Direction after Aging Treatment

After an aging treatment equivalent to baking at 210° C. for 10 minutes, a JIS No. 5 tensile test piece was sampled in the transverse direction and was tested in accordance with JIS Z 2241 to evaluate the tensile strength and the elongation (total elongation).

Average Ferrite Grain Size

A cross section in the rolling direction was buried, polished, and etched with Nital to expose the grain boundaries. In accordance with JIS G 0551, the average crystal grain sizes were measured by a linear intercept method. The average ferrite grain size was thus evaluated.

Dislocation Density

The dislocation density was measured by the Williamson-Hall method. Specifically, half-value widths of the diffraction peaks assigned to (110), (211) and (220) planes were measured at a depth of ¼ sheet thickness and the results were corrected using the half value widths obtained with respect to a strain-free Si sample. The strain 6 was determined. The dislocation density (m⁻²) was evaluated based on ρ14.4ε²/(0.25×10⁹)².

EOE Rivet Formability

After an aging treatment equivalent to baking at 210° C. for 10 minutes, a rivet for the attachment of an EOE tab was formed to evaluate the rivet formability. The rivet was formed by 3-step pressing. The steel sheet was bulged and was thereafter shrunk (reduced in diameter) to form a cylindrical rivet 4.0 mm in diameter and 2.5 mm in height. The rivet formability was evaluated as “x” when wrinkles or cracks had occurred on the rivet surface, and as “O” when the surface was free from wrinkles or cracks.

Can Body Flangeability

After an aging treatment equivalent to baking at 210° C. for 10 minutes, the steel sheet was seam welded to form a can body 52.8 mm in outer diameter. The end portions were necked in to an outer diameter of 50.4 mm and were thereafter flanged to an outer diameter of 55.4 mm. The presence or absence of flange cracks was evaluated. The can body formed was of 190 g beverage can size. The welding was performed along the steel sheet rolling direction. The necking-in was carried out by a die-necking process, and the flanging by a spin-flanging process. The flangeability was evaluated as “x” when the flanged portions had been cracked, and as “O” when there was no cracks.

Strength of Cans

Cans were fabricated by sealing lids to those samples which had been successfully necked-in and flanged. The strength of the cans was measured by a dent test. An indenter having a tip radius of 10 mm and a length of 42 mm was pressed against the center of the can body opposite to the weld, and the load which caused the can body to be buckled was measured. The strength of the cans was evaluated as good “O” when the load was 70 N or more, and as “x” when the load was less than 70 N. The hyphens indicate that the steel sheet had been cracked during the flanging and the fabrication of a can failed.

The results are described in Table 3. In all Inventive Examples, the steel sheets achieved excellent strength and formability, with the tensile strength being not less than 530 MPa, the elongation being not less than 7%, the ferrite grain size being not more than 7.0 μm, and the density of dislocations at a depth of ¼ sheet thickness being 4.0×10¹⁴ m⁻² to 2.0×10¹⁵ m⁻². In contrast, the steel sheets of Comparative Examples were poor in one or more of these characteristics.

TABLE 1 (mass %) Steel C Si Mn P S Al N Remarks A 0.034 0.01 0.24 0.012 0.011 0.015 0.0155 Inv. Ex. B 0.020 0.02 0.30 0.014 0.010 0.012 0.0144 Inv. Ex. G 0.039 0.01 0.14 0.009 0.013 0.010 0.0163 Inv. Ex. D 0.035 0.01 0.58 0.013 0.009 0.018 0.0122 Inv. Ex. E 0.028 0.01 0.70 0.008 0.008 0.008 0.0175 Inv. Ex. F 0.078 0.01 0.35 0.010 0.009 0.019 0.0132 Inv. Ex. G 0.083 0.01 0.26 0.013 0.012 0.015 0.0132 Comp. Ex. H 0.005 0.01 0.22 0.011 0.010 0.016 0.0149 Comp. Ex. I 0.036 0.01 0.25 0.009 0.012 0.012 0.0191 Comp. Ex. J 0.031 0.01 0.35 0.010 0.008 0.090 0.0038 Comp. Ex. K 0.034 0.01 0.35 0.013 0.010 0.023 0.0149 Inv. Ex. L 0.042 0.01 0.26 0.015 0.011 0.016 0.0126 Inv. Ex. M 0.022 0.01 0.61 0.013 0.009 0.018 0.0153 Inv. Ex. N 0.035 0.01 0.51 0.012 0.010 0.017 0.0148 Inv. Ex.

TABLE 2 Thickness of Primary cold Secondary Slab heating Hot rolling Hot-rolled sheet hot-rolled rolling Annealing cold rolling Sheet Steel temp. finish temp. coiling temp. sheet reduction temp. reduction thickness sheet No. Steel ° C. ° C. ° C. mm % ° C. % mm Remarks 1 A 1230 870 540 2.3 89.6 660 12 0.21 Inv. Ex. 2 B 1230 890 580 2.6 88.9 620 10 0.26 Inv. Ex. 3 C 1210 900 500 2.0 90.2 630 8 0.18 Inv. Ex. 4 D 1180 820 620 1.8 88.9 670 15 0.17 Inv. Ex. 5 E 1260 850 520 2.1 91.1 690 20 0.15 Inv. Ex. 6 F 1220 890 610 2.0 89.8 680 12 0.18 Inv. Ex. 7 G 1240 880 550 2.0 89.8 680 12 0.18 Comp. Ex. 8 H 1195 870 540 1.8 90.7 650 10 0.15 Comp. Ex. 9 I 1240 880 590 2.2 88.8 660 15 0.21 Comp. Ex. 10 J 1250 850 550 2.4 90.4 650 9 0.21 Comp. Ex. 11 A 1220 970 560 2.3 92.3 640 15 0.15 Comp. Ex. 12 A 1220 780 560 2.3 90.8 640 15 0.18 Comp. Ex. 13 A 1230 860 680 2.0 90.9 670 12 0.16 Comp. Ex. 14 B 1150 870 600 1.6 80.0 670 10 0.20 Comp. Ex. 15 B 1210 840 580 2.3 87.7 740 8 0.26 Comp. Ex. 16 B 1210 840 540 2.3 87.7 550 8 0.26 Comp. Ex. 17 B 1190 890 610 0.18 91.2 625 5 0.15 Comp. Ex. 18 B 1190 890 610 2.6 90.1 625 30 0.18 Comp. Ex. 19 K 1210 870 580 2.0 90.3 670 12 0.17 Inv. Ex. 20 L 1190 900 560 2.0 90.0 650 10 0.18 Inv. Ex. 21 M 1220 880 530 2.0 90.0 650 10 0.18 Inv. Ex. 22 N 1220 880 530 2.0 90.0 650 10 0.18 Inv. Ex.

TABLE 3 Tensile Dislocation Average ferrite Strength of Evaluation of Steel Solute N strength Elongation density grain size EOE rivet cans strength of sheet No. Steel mass % MPa % 10¹⁴ m⁻² μm formability Fangeability N cans Remarks 1 A 0.0148 580 11 7.1 5.6 ∘ ∘ 155 ∘ Inv. Ex. 2 B 0.0140 550 12 6.5 5.2 ∘ ∘ 260 ∘ Inv. Ex. 3 C 0.0159 555 14 5.5 5.1 ∘ ∘ 98 ∘ Inv. Ex. 4 D 0.0106 620 9 10.2 5.9 ∘ ∘ 90 ∘ Inv. Ex. 5 E 0.0166 675 7 14.6 5.4 ∘ ∘ 75 ∘ Inv. Ex. 6 F 0.0105 590 7 8.0 6.0 ∘ ∘ 102 ∘ Inv. Ex. 7 G 0.0121 600 4 6.8 5.6 x x — — Comp. Ex. 8 H 0.0136 480 13 6.4 7.9 x ∘ 51 x Comp. Ex. 9 I 0.0186 640 4 11.2 6.3 x x — — Comp. Ex. 10 J 0.0019 490 9 5.8 6.7 x x — — Comp. Ex. 11 A 0.0145 515 7 9.7 7.5 ∘ ∘ 55 x Comp. Ex. 12 A 0.0146 600 5 11.7 6.3 x x — — Comp. Ex. 13 A 0.0115 520 9 6.9 7.3 ∘ ∘ 63 x Comp. Ex. 14 B 0.0122 523 10 6.3 8.6 x x — — Comp. Ex. 15 B 0.0142 518 11 4.9 8.1 x x — — Comp. Ex. 16 B 0.0139 580 3 7.6 4.3 x x — — Comp. Ex. 17 B 0.0126 480 15 2.8 6.2 ∘ ∘ 50 x Comp. Ex. 18 B 0.0125 690 4 26.1 6.1 x x — — Comp. Ex. 19 K 0.0112 545 9 7.8 4.7 ∘ ∘ 83 ∘ Inv. Ex. 20 L 0.0106 540 8 5.8 6.2 ∘ ∘ 95 ∘ Inv. Ex. 21 M 0.0139 620 11 6.8 5.0 ∘ ∘ 101 ∘ Inv. Ex. 22 N 0.0123 595 13 6.3 5.4 ∘ ∘ 96 ∘ Inv. Ex. 

1. A high-strength steel sheet having a chemical composition comprising: C: 0.010% to 0.080%, by mass° ₉:,, Si: 0.05% or less, by mass %; Mn: 0.10% to 0.70%, by mass %; P: 0.03% or less, by mass %; S: 0.020% or less, by mass %; Al: 0.005% to 0.070%, by mass %; and N: 0.0120% to 0.0180%, by mass %; the balance being Fe and inevitable impurities, wherein: a content of nitrogen in the form of solute nitrogen is 0.0100% or more, an average ferrite grain size of the steel sheet is 7.0 μm or less, a density of dislocations at a depth of ¼ sheet thickness from the surface of the steel sheet is 4.0×10¹⁴ m⁻² to 2.0×10¹⁵ m⁻², a tensile strength in a transverse direction, perpendicular to a rolling direction, after aging treatment of the steel sheet is 530 MPa or more, and an elongation in the transverse direction after the aging treatment is 7% or more.
 2. A method for manufacturing the high-strength steel sheet described in claim 1, the method comprising: a hot rolling step of (i) heating a slab at a heating temperature of 1180° C. or more, (ii) rolling the slab with a hot rolling finish temperature of 820 to 900° C. to produce a hot-rolled steel sheet, and (iii) coiling the hot-rolled steel sheet at a coiling temperature of 640° C. or less, a primary cold rolling step of (i) pickling the hot-rolled steel sheet and (ii) cold rolling the hot-rolled steel sheet with a rolling reduction of 85% or more, an annealing step of annealing the primarily cold-rolled steel sheet at 620° C. to 690° C., and a secondary cold rolling step of secondarily cold rolling the annealed steel sheet with a rolling reduction of 8 to 20%.
 3. The high-strength steel sheet according to claim 1, wherein the chemical composition comprises Al from 0.005% to 0.020%, by mass %.
 4. The high-strength steel sheet according to claim 1, wherein the chemical composition comprises N from more than 0.0130% to 0.0180%, by mass %,
 5. The high-strength steel sheet according to claim 1, wherein the content of nitrogen in the form of solute nitrogen is 0.0120% or more.
 6. The high-strength steel sheet according to claim 1, wherein the chemical composition comprises Mn from more than 0.50% to 0.70%, by mass %.
 7. The method according to claim 2, wherein, during the hot rolling step, the coiling temperature is 600° C. or less.
 8. The method according to claim 2, wherein, during the annealing step, annealing the primarily cold-rolled steel sheet at 620° C. to 680° C. 